1. Field of the Invention
This invention relates to an alloy steel composition, in particular, a low-maganese alloy steel composition suitable for cryogenic applications and a method for preparing the composition.
2. Description of the Prior Art
Due to dwindling natural gas supplies in this and other countries there is considerable interest in containment vessels for safely transporting liquefied natural gas (LNG) by ship and other transport. Because the boiling temperature of natural gas is in the cryogenic (generally below about -80.degree. to -100.degree. C.) range, LNG containers must be designed to avoid breakage due to pressure and crack development over a broad temperature range. There is also the danger of a catastrophic explosion or fire, should the containment vessel fail.
At cryogenic temperatures, ordinary steel alloys lose much of their resilence and become very brittle. A denominator of the steel alloys commonly specified for structural applications at LNG and lower temperatures is a relatively high content of nickel. The nickel contributes significantly to good low temperature properties; but, is a relatively scarce metal and, thus, adds substantially to the cost. Recently, lower (5-6%) Ni steels have been introduced to reduce cost.
Storage systems for other liquefied gases, particularly nitrogen, oxygen, and liquid air, are also a significant market for cryogenic alloys. This market is different than that for LNG in that the safety standards are less stringent and a larger number of alloys compete with more emphasis placed on materials cost.
Of the common alloying elements in steels, manganese is considered the most attractive substitute for nickel. Manganese is readily available, and, thus, relatively inexpensive, and has a metallurgical similarity to nickel in its effect on the microstructures and phase relationships of iron-based alloys. Therefore, there has been considerable interest in the potential of Fe-Mn alloys for cryogenic use.
Fe-12 Mn (12% manganese) alloys have been made tough at 77.degree. K. (-196.degree. C.) by several methods: (1) a cold work plus tempering treatment, (2) controlled cooling through the martensite transformation, and (3) the addition of a minor amount of boron. However, although manganese is less expensive than nickel, it also adds to the cost of the steel, and, therefore, a lower manganese content would be advantageous.